Generally, a typical High Intensity Discharge (HID) lamp has a fixed lamp voltage load. Under some circumstances, it is desired to deliver to the lamp more power than normal. For example, in some lamp applications it is desired to provide fast lamp warm-up. The amount of power delivered to an HID lamp is the product of the lamp voltage and the current supplied by the ballast. The current available from a conventional electronic ballast is typically limited by its current carrying switching components. Therefore, using such a current limited ballast with a conventional fixed lamp voltage load, it has not been possible heretofore to supply more power to the lamp. Although the current supplied by the ballast can be increased by providing higher current switching components, higher current switching components are more expensive than conventional current limited ballasts. The present invention increases lamp voltage, using a conventional current limited ballast, by bending the arc in the arc tube to increase the length of the arc. For example, arc length can be increased by injecting acoustic frequencies into the waveform of the power supplied to the lamp to bend the arc.
Bending the arc to increase its length is contrary to conventional treatment of the arc in an arc tube of a discharge lamp. In particular, in conventional lamp applications provided heretofore, whenever efforts have been made to influence the arc, such efforts have involved straightening or otherwise stabilizing and centering the arc. For example, U.S. Pat. No. 5,134,345 issued on Jul. 28, 1992 to El-Hamamsy et al. illustrates a method of detecting arc instabilities in an HID lamp and changing the drive frequencies that cause them thereby avoiding acoustic frequencies that cause destabilizing phenomena.
In U.S. Pat. No. 5,306,987 issued on Apr. 26, 1994 to Dakin et al. reference is made to stabilization of HID lamps by modulating the drive signal with acoustic resonant band frequencies. A similar method of centering the arc in discharge lamps is illustrated in U.S. Pat. No. 5,198,727 which issued on Mar. 30, 1993 to Allen et al. This patent illustrates centering the arc by the "acoustic perturbations" induced by the frequency of the drive signals. Such acoustic perturbations compel the gas or vapor movement patterns within the arc tube to counter the gravity-induced convection.
U.S. Pat. No. 5,684,367 which issued on Nov. 4, 1997 to Moskowitz et al. illustrates a system for and method of operating a discharge lamp, and in particular, of stabilizing and controlling the characteristics of discharge lamps by amplitude-modulating the input AC power wave with a periodic waveform and/or pulse wave to control stabilization and color characteristics. This patent is commonly owned with the instant application and is incorporated herein by reference.
In U.S. Pat. No. 5,047,695 which issued on Sep. 10, 1991 to Allen et al., a method and ballast circuit is illustrated for operating fluorescent, mercury vapor, sodium and metal halide lamps in a DC mode. Power modulation for creating acoustic pressure waves for arc straightening is referred to in this patent. The lamp illustrated therein is operated with a selectable amount of ripple imposed to provide for acoustically straightening the arc between the lamp electrodes. A related patent is the aforementioned 5,198,727 patent.
All of the foregoing patents relate to straightening or otherwise stabilizing and centering the arc in an arc tube in a discharge lamp. In contrast, the present inventors have developed a new method and system for operating a discharge lamp wherein more power may be supplied to a fixed lamp voltage load by bending the arc to increase the arc length and therefor the lamp voltage. None of the foregoing references illustrate this feature. Bending of the arc to increase arc length may be effected, for example, using acoustic frequencies, and in particular by exciting particular acoustic resonances in the gas in the arc tube thereby causing the arc to bend.